The present invention relates to a calibration apparatus and method for use with an antenna array. In particular, but not exclusively, the present invention is applicable to phased antenna arrays for use in cellular telecommunication networks using beam steering.
With currently implemented cellular telecommunication networks, a base transceiver station (BTS) is provided which transmits signals intended for a given mobile station (MS), which may be a mobile telephone, throughout a cell or cell sector served by that base transceiver station. However, in space division multiple access systems, the base transceiver station will only transmit a signal in a beam direction from which a signal from the mobile station is received. In other words, the base transceiver station does not transmit a signal throughout the cell or cell sector. The base transceiver station is also able to determine the direction from which the signals from mobile stations are received. SDMA is one example of beam steering. Other types of beam steering are also known.
To direct the beam in a given direction, the base transceiver station will generally have a phased antenna array. Typically, such an antenna array will comprise a number of antennas, for example 4 or 8 antennas, arranged with a spacing of, for example, one half of a wavelength therebetween. A signal to be transmitted is supplied to each of the antennas but with different relative phases. Depending on these phase differences, there will be constructive interference in the desired beam direction and destructive interference in the undesired directions. In order to ensure that the beam is provided only in the desired direction, it is important to ensure that the signal to be transmitted is provided to each of the antennas with the correct relative phase shift. In other words the same signal is applied to each of the antennas but with different relative phases. Likewise, in order to determine the direction from which a signal has been received, it is necessary to analyse the relative phase shifts of the signal received at each of the antennas. Typically, the processing means for generating the relative phase shifts for signals to be transmitted and for analysing the relative phase shifts of received signals is some distance from the antennas. Accordingly, differences in the length of the cabling between each antenna and the processing means as well as differences in temperature in the different cabling can adversely effect the relative phases. If this occurs, then the beam may not be generated in the desired direction. In the case of received signals, it will not be possible to accurately determine the direction from which a signal has been received.
Calibration circuitry can be used to ensure that the beams produced by the antenna array are as desired by the base station. The circuitry should be placed close to the antenna. This is to ensure accuracy. The antennas in base stations tend to be located at the top of a mast and therefore make the calibration circuitry difficult to maintain and replace. Furthermore, if the calibration circuitry is damaged or fails to operate correctly, there is an increased likelihood of the base station failing to operate. This would put unnecessary pressure on the network to service the subscribers who would normally be serviced by the inoperable base station. It may leave an area, and the subscribers within that area, without any network coverage for an extended period of time. Base stations which use beam steering can service a relatively large number of people at the same time. To have such a base station out of action would adversely affect a network. Some base stations may be located in countries where severe winters mean that the base station can not be accessed during winter and repaired. To have a base station non operational for this length of time is clearly disadvantageous.
It is therefore an aim of embodiments of the present invention to address this problem.
According to a first aspect of the present invention, there is provided a system for use with an antenna array having a plurality of antennas, said system comprising a first calibration arrangement for calibration of signals of said antenna array; a second calibration system for calibration of signals of said antenna array; and selection means for selecting one of said calibration arrangements for calibrating signals of said antenna array.